Methods: Giving Objects Behaviour

Why this matters

A method is a function that lives inside a class and works on one object’s data. This is the heart of object-oriented programming: the data and the code that changes it travel together, so the rest of your program just asks the object to do things.

The idea

class Counter:
    def __init__(self):
        self.count = 0

    def increment(self, by=1):      # a method: first parameter is self
        self.count += by

    def reset(self):
        self.count = 0

    def is_zero(self):
        return self.count == 0

c = Counter()
c.increment()
c.increment(5)
print(c.count)          # 6
c.reset()

Calling c.increment(5) is the same as Counter.increment(c, 5): c becomes self. Inside a method, use self.name to read or change the object’s data, and self.other_method() to call another method. A method with no return returns None.

Try it

Add a method double() to Counter that doubles the count. Create two counters and check they do not affect each other.

Your turn

Write a class Cart (a shopping cart) with these methods:

• add(name, price, qty=1): add items. Adding a name that is already in the cart increases its quantity and keeps the original price.
• remove(name): remove an item completely. A name that is not in the cart raises KeyError.
• total(): the total price, rounded to 2 decimals.
• count(): how many things are in the cart in all (the sum of the quantities).
• most_expensive(): the name of the item with the highest unit price, or None for an empty cart.

Recap

• A method is a function in a class; its first parameter is self.
• Methods read and change the object’s attributes through self.
• Let the object manage its own data; callers just call methods.

Common mistakes

• Forgetting self. when using an attribute inside a method: NameError: name 'items' is not defined.
• Calling a method without parentheses (cart.total instead of cart.total()) gives a method object, not the answer.
• Mutating a class-level dictionary or list, which every object shares.